The limiting oxygen index (LOI) test is one of the most widely used small-scale flammability screening methods for plastic materials, quantifying the minimum oxygen concentration required to just sustain combustion. This article presents a complete plastic oxygen index assessment solution covering internationally recognized standards such as ISO 4589-2, ASTM D2863, and GB/T 2406.2, together with sample preparation, test procedures, result interpretation, and typical industrial applications.
Technical Overview
The limiting oxygen index (LOI), also called the oxygen index (OI), is defined as the minimum concentration of oxygen, expressed as volume percent, in a flowing mixture of oxygen and nitrogen that will just support flaming combustion of a vertically mounted plastic specimen after ignition with a small flame. Because ambient air contains approximately 20.9% oxygen by volume, the LOI value provides an intuitive benchmark: materials with an LOI of 21 or lower will continue to burn in ordinary air, while materials with higher LOI values tend to self-extinguish once the ignition source is removed.
The oxygen index method is highly reproducible and produces a continuous numerical result rather than a simple pass/fail classification. This makes it exceptionally valuable for ranking flame-retardant formulations, supporting research and development of halogen-free compounds, verifying batch-to-batch consistency in production, and providing regulatory evidence of flame retardancy. The LOI test is a small-scale screening method and is typically used alongside other flammability assessments such as the UL 94 burning tests (IEC 60695-11-10 and IEC 60695-11-20), the glow-wire tests of IEC 60695-2 series, and heat release analysis per ISO 5660 for a complete fire-safety profile.
- ISO 4589-1: general guidance for oxygen index testing
- ISO 4589-2 and EN ISO 4589-2: ambient-temperature limiting oxygen index test for plastics
- ISO 4589-3: elevated-temperature oxygen index and temperature index determination
- ASTM D2863: standard test method for measuring the minimum oxygen concentration to support candle-like combustion of plastics
- GB/T 2406.2: Chinese national standard, technically aligned with ISO 4589-2
- JIS K 7201-2: Japanese standard corresponding to ISO 4589-2
Testing Samples
Specimen geometry has a direct influence on oxygen index results, so the assessment scheme defines sample configurations according to material form. All specimens must be representative of the material as supplied or as processed, free from surface contamination, and conditioned before testing.
Rigid Self-Supporting Moldings
Injection-molded or compression-molded bars, typically 80 mm to 150 mm long, 10 mm wide, and 4 mm thick, represent the default specimen configuration for thermoplastics and thermosets. Test plates may be prepared in accordance with ISO 293, ISO 294, or ISO 20753 as appropriate for the material.
Sheet and Thin Materials
Rigid sheets with a thickness of less than 4 mm are tested at their as-supplied thickness. Because LOI values generally increase with specimen thickness, the tested thickness must always be reported, and comparison between materials is only valid at equal thickness.
Flexible Films and Tapes
Thin, limp materials that cannot support themselves are rolled onto a support rod or mounted in special holders as specified in the applicable standard, ensuring a reproducible specimen geometry for flexible films up to approximately 100 micrometers in thickness.
Cellular and Foam Materials
Rigid and flexible foams are tested using enlarged cross-sections, commonly 80 mm to 150 mm long with a nominal 10.5 mm by 10.5 mm cross-section, to account for the lower density and faster burning behavior of cellular structures.
Specimen Preparation and Conditioning
A minimum of 15 specimens, with 20 recommended, should be prepared for each test condition. Specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 88 hours in accordance with ISO 291, unless the material specification or client agreement requires otherwise. Each specimen is marked with a reference line 50 mm below the top edge to define the burning-extent criterion, and the machine direction or molding orientation is recorded for anisotropic materials.
Testing Items
The core deliverable of the assessment is the ambient-temperature limiting oxygen index expressed in volume percent of oxygen, supported by quantitative and observational data that characterize burning behavior. A comprehensive program can include the following items:
- Ambient-temperature limiting oxygen index (LOI) per ISO 4589-2, ASTM D2863, GB/T 2406.2, or JIS K 7201-2
- Elevated-temperature oxygen index and temperature index per ISO 4589-3
- Burning behavior observations: ease of ignition, afterflame time, afterglow, burning extent, dripping, and char formation
- Thickness-series LOI profiling to support material datasheets and design margins
- Comparative LOI evaluation before and after heat aging, UV exposure, or humidity conditioning
- Batch-to-batch quality control verification for flame-retardant compounds and masterbatches
- Formulation benchmarking for halogen-free and traditional flame-retardant development programs
Testing Methods
Test Apparatus
The oxygen index apparatus consists of a vertical heat-resistant glass chimney of at least 450 mm height, a gas mixing and metering system supplying oxygen and nitrogen with calibrated flow control, and a small-diameter igniter tube producing a low-energy diffusion flame fueled by an unpolluted hydrocarbon gas such as propane. The specimen is held vertically on a clamp with its upper end at least 100 mm below the top of the chimney, and the total gas flow is regulated to give a nominal linear velocity of 40 mm/s through the column so that the specimen burns in a gently laminar atmosphere.
Standard Test Procedure
The test follows the candle-burning principle established by ISO 4589-2 and ASTM D2863. The procedure below summarizes the ISO method; ASTM D2863 follows an essentially equivalent sequence with minor procedural variants.
- Condition, measure, and mark each specimen, then mount it vertically in the chimney.
- Set an initial oxygen concentration based on experience or preliminary trials and allow the gas flow to stabilize.
- Apply the igniter flame to the top of the specimen for 5 seconds, withdraw it, and repeat the application as necessary within the number of attempts allowed by the standard.
- Record whether the specimen satisfies the burning criterion: continuous burning for 180 seconds or longer, or flame spread reaching the reference mark 50 mm below the top edge.
- Adjust the oxygen concentration using a bracketing sequence, typically beginning with coarse steps of 1% oxygen and refining to steps of 0.2% oxygen, until the boundary between burning and self-extinguishing is bracketed.
- Calculate the LOI from the mean of the limiting oxygen concentrations obtained in the final bracketing series and report the result to the nearest 0.1% oxygen by volume.
Result Interpretation
LOI results are conventionally interpreted against ambient air oxygen content. The following guidance reflects common industry practice, although contractual or regulatory specifications always take precedence.
- LOI below 21%: readily flammable, will continue to burn in normal air
- LOI of 21% to 25%: slowly burning with a tendency toward self-extinguishing
- LOI of 25% to 28%: generally considered flame retardant for many polymer applications
- LOI of 28% and above: high flame retardancy; specifications for wire and cable compounds, rail components, and mining products frequently require 28% to 32% or higher
Because LOI depends on specimen thickness, temperature, orientation, and conditioning history, the test report must state all relevant parameters. Accredited third-party laboratories ensure traceable calibration of gas flow and oxygen analysis, which is essential for inter-laboratory agreement, and can issue reports suitable for supplier qualification, certification support, and regulatory submissions across ISO, ASTM, EN, GB, and JIS market frameworks.
Applications
Oxygen index testing supports material qualification and fire-safety compliance across a broad range of industries in which plastics are used in safety-critical roles.
- Wire and cable: flame-retardant insulation and sheathing compounds, where LOI is specified alongside vertical burning tests of the IEC 60332 series
- Electrical and electronic equipment: enclosures, connectors, and engineering components evaluated together with UL 94 and glow-wire requirements
- Railway applications: interior and infrastructure components referenced within the EN 45545-2 fire-safety requirement framework
- Construction: panels, pipes, flooring, and insulation materials supporting classification activities under EN 13501-based systems
- Automotive, aerospace, and marine interiors: screening of seat, panel, trim, and textile-laminate materials for fire-performance programs
- Mining: conveyor belting, cable jackets, and ventilation ducting materials requiring elevated LOI values
- Polymer and masterbatch manufacturing: incoming resin control, formulation optimization, and batch release of flame-retardant grades
- Research institutions: academic and industrial studies of combustion behavior and flame-retardant mechanisms
FAQ
What oxygen index value is considered flame retardant?
As general guidance, materials with an LOI of 25% or above are widely regarded as flame retardant, while values of 28% or higher indicate high flame retardancy. However, the acceptance threshold depends on the application: cable compounds and rail components are often specified at 28% to 32% or above, so the governing product standard or purchase specification should always be consulted.
How does the LOI test relate to UL 94 ratings?
The two methods measure different aspects of flammability. LOI quantifies the relative ease of sustaining combustion in a controlled atmosphere and produces a numerical value, whereas UL 94 (IEC 60695-11-10 and IEC 60695-11-20) evaluates ignition, flame spread, afterflame, and flaming drips in normal air. A higher LOI usually correlates with better UL 94 performance, but the correlation is not absolute, and most certification programs require both types of data.
Why does the LOI result change with specimen thickness?
Thicker specimens act as a larger heat sink and lose combustion heat more readily to the surrounding material and the flow gas, so burning is harder to sustain and the measured LOI rises. For this reason, LOI results are only comparable at identical thickness, and the assessed thickness must always be stated on the report.
How many specimens are required for one determination?
A complete determination typically consumes at least 15 specimens, and preparing 20 specimens is recommended to allow for the coarse exploration steps of the bracketing procedure and for retesting when results fall near the burning criterion.
Does a high LOI guarantee safety in a real-scale fire?
No. The oxygen index test is a small-scale screening method designed for material comparison and quality control. Real fire performance also depends on heat release, smoke production, and flame spread, which are evaluated by methods such as ISO 5660 cone calorimetry and full-scale or intermediate-scale fire tests. LOI should form one element of a broader fire-hazard assessment.
Can the oxygen index be measured at elevated temperature?
Yes. ISO 4589-3 extends the method to elevated temperatures, allowing determination of an oxygen index at a specified temperature or a temperature index, which is the temperature at which the material exhibits a defined LOI, typically 20.9%. This is particularly relevant for materials used in warm environments where flame retardancy decreases with temperature.
Are ISO, ASTM, GB, and JIS oxygen index results mutually recognized?
ISO 4589-2, EN ISO 4589-2, ASTM D2863, GB/T 2406.2, and JIS K 7201-2 are technically harmonized in principle and use the same candle-burning concept. Minor differences exist in apparatus details, procedural variants, and reporting, so the exact standard required by the target market or specification should be confirmed before testing. Accredited third-party laboratories can perform multi-standard testing on identical specimen sets to support global market access.